1. Field of the Invention
The present invention is directed to a system and method for reducing nitrogen oxides (NOx) in a fossil fuel combustion exhaust. More particularly, it relates to a system and method for reducing NOx in the combustion exhaust from a turbocharged engine.
2. Background of the Related Art
The combustion of fossil fuels results in the formation of nitrogen oxides (NOx), a pollutant that leads to smog and acid rain, especially in urban environments. Most of the NOx formed during the combustion process is the result of two oxidation mechanisms. The first, referred to as thermal NOx, is formed by the reaction of nitrogen at elevated temperatures with oxygen in the combustion air, said oxygen being present in excess of the amount required for stoichiometric combustion. The presence of excess oxygen is required for the combustion of all fossil fuels to minimize the generation of other pollutants, such as carbon monoxide. The second involves the oxidation of nitrogen that is chemically bound in the fossil fuel, and is referred to as fuel NOx.
For example, in a coal fired furnace, thermal NOx typically represents about 25% and fuel NOx about 75% of the total NOx generated. However, for cyclone boilers or other boilers that operate at very high temperatures, thermal NOx can be considerably higher than fuel NOx. Therefore, flame temperature, the residence time at temperature, and the degree of fuel/air mixing, along with the nitrogen content of the fossil fuel and the quantity of excess air used for combustion, will usually determine the NOx levels in the exhaust gas.
Combustion modifications, such as strict management of the mixing of fuel and air, temperature reduction/optimization, and measures which reduce turbulence can minimize NOx formation in some combustion devices.
In internal combustion engines, such as reciprocating engines, exhaust gas recirculation (EGR) has been used to reduce NOx emissions. This technique reduces the amount of oxygen involved in the combustion, by directing a portion of the exhaust gases back into the intake of the engine. The oxygen in the exhaust gases has already been used by the engine and the total amount of oxygen entering the combustion zone of the engine is reduced by since the combustion intake is a mixture of the exhaust gases containing little oxygen with the fresh intake air. Since there is less oxygen to react with, less NOx are formed. Also, the exhaust gases do not participate in the combustion process which lowers the peak operating temperature and results in less thermal NOx formation.
Although EGR reduces NOx emissions, the technique also increases particulate emissions and the chances that certain apparatus, such as heat exchangers, will be fouled by the particulates. Furthermore, the substitution of the intake fuel/air mixture with exhaust gases reduces the power generated by the engine and lowers engine performance. Reduced power generation poses a significant problem to the end users that depend on reciprocating engines to generate electrical power, such as electrical power companies which rely on reciprocating engines, especially during peak load times, to enhance power distribution quality and reliability. Furthermore, there is increasing demand for reciprocating engines which are powerful enough to satisfy substantial power needs not being supplied by power companies, since using reciprocating engines offers a faster, less expensive alternative to constructing large, central power plants and high-voltage transmission lines.
Regulatory requirements for controlling NOx emissions, such as those pertaining to ground-level ozone issues, are advancing at a rapid pace. Emission limits are becoming more stringent while at the same time the need for devices that produce NOx emissions as a byproduct is increasing.
For such reasons, it is anticipated that the use of post-combustion controls via selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) will significantly increase to achieve greater removal of NOx than is obtainable by combustion modification alone.
In SCR, a vessel containing a catalyst, such as vanadium/titanium formulations (e.g., V2O5 stabilized in a TiO2 base), activated carbon, zeolite materials, compositions of active metals and support materials or the like, is installed downstream of the combustion zone. A reducing agent, typically ammonia (NH3), is injected into the flue gas before it passes over the fixed-bed catalyst. The catalyst promotes a reaction between NOx and the ammonia to form nitrogen (N2) and water vapor.
SNCR involves the injection of a reducing agent, typically either urea or ammonia, with or without various chemical additives, into the combustion exhaust gases at temperatures at which the NOx to N2 reaction is favored.
SCR is similar to SNCR in that it uses a reducing agent applied to the effluent gas from the combustion zone to convert NOx emissions to elemental nitrogen and water. The key difference between the two is the presence of the catalyst in the SCR system, which accelerates the chemical reactions. The catalyst is needed because SCR systems operate at much lower temperatures than do the SNCR; typical temperatures for SCR are 340 to 575° C. (650 to 1100° F.), compared with 870 to 1,200° C. (1,600 to 2,200° F.) for SNCR.
SNCR and SCR can be used together, and either process can be used in conjunction with Low NOx Burners (LNBs), which are designed to control the mixing of fuel and air to achieve what amounts to staged combustion. While all these NOx control processes reduce NOx emissions to varying degrees, they all have certain technical and economic limitations and disadvantages.
The present invention is directed to a system and method which reduces NOx and through advantageous use of components in the combustion process, among other things, eliminates some of the disadvantages in the prior art NOx control systems and methods, primarily in prior art SNCR and SCR operations.